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We all know that plants have seeds. If you plant a seed from a fruit or vegetable you will get the same, or similar fruit or vegetable. So where do the seeds come from for a seedless watermelon?

There are seeds for seedless watermelons. However to have the vines bear fruit we plant watermelon seeds from watermelons with seeds to pollinate the seedless variety.

Here’s the best and most concise explanation I could find. If you would rather skip the science class, feel free to scroll down to our newest recipe, Agua Fresca de Sandia.

The first seedless watermelons were produced in 1939 by Kihara and Nishiyama, researchers at the Japanese National Institute of Genetics.

A normal watermelon is diploid, which means it has two sets of chromosomes (the thread-like structures that carry an organism’s genes). Normally, when one plant pollinates another, the embryo plants in the resulting seeds contain one set of chromosomes from one parent and one from the other, giving them a complete set of two.

But there’s a chemical called colchicine, an alkaloid extracted from the autumn crocus and some other plants that can cause the chromosomes in dividing plant cells to double. By applying colchicine to the terminal buds of watermelon plants, Kihara and Nishiyama were able to produce offspring with were tetraploid–they had four sets of chromosomes instead of just two.

They then mated the tetraploid plants with ordinary diploid plants. Since the offspring received two sets of chromosomes from one parent and only one from the other, they were triploid–they had three sets of chromosomes. They were also sterile: they’d produce flowers, but no seeds.

Of course, since fruit exists to house seeds, the triploid plants also had no reason to produce watermelons. Fortunately, they could be tricked into it: if their flowers were pollinated by a fertile plant with an even number of chromosomes, they acted as if they’d been fertilized, even though they really hadn’t, and developed fruit (which isn’t, strictly speaking, seedless, but the pale, soft seeds it does contain can be eaten instead of having to be spit out.)

In the field, seedless watermelon seeds are produced by sandwiching a row of tetraploid plants between two rows of diploid plants. Seedless watermelons, in turn, are produced by sandwiching a row of seedless watermelon plants between rows of regular watermelon plants, because the triploid plants have to be pollinated by diploid or tetraploid plants to produce fruit.